Send leave to most recently entered window when modal dialog is shown.

Send leave to most recently entered window when modal dialog is shown.

If a modal dialog was shown as a response to button click, the button
retained its hover highlight, because it didn't get leave event.

Fixed by tracking the most recently entered window and sending a leave
to it when modal dialog is shown that blocks it.

Also modified tst_QGuiApplication::modalWindow() autotest to check
for enters and leaves.

Task-number: QTBUG-27644
Change-Id: I387647e18a762a39d523e3df31221b9583a39f9d
Reviewed-by: Samuel Rødal <samuel.rodal@digia.com>